ahasja wrote:
Sometimes I drink Matcha. When I try different ones, I always like Matchas that are called "mukashi". What does that exactly mean?

It has a literal and figurative meaning.

Mukashi means something like "olden days" or "long ago". In Japan, the phrase mukashi mukashi is often used at the beginning of a story, in much the same way we begin a tale with "Once upon a time".

When it appears in the name of a matcha, mukashi suggests that the matcha is of sufficient high quality to be used for koicha, "thick tea". Only the higher, sweeter grades can be used in this way, as the lower grades are too bitter to be used at this concentrated strength.

Not all koicha-grade matcha-s has a name beginning with mukashi, but most of them do.